In a typical cellular wireless network, a geographic area is divided into cell sectors. Each cell sector defines a geographic area in which wireless terminals (such as cellular telephones, personal digital assistants (PDAs) and/or other devices) operate. The wireless network normally has a base transceiver station (BTS) assigned to one or more cell sectors. The BTS produces a radio frequency (RF) radiation pattern over the one or more cell sectors. The RF radiation pattern allows the wireless terminals located in the one or more cell sectors to exchange signals with the BTS over an air interface.
The wireless network typically has a plurality of BTSs. The plurality of BTSs communicates concurrently with a base station controller (BSC) that aggregates signals received from the plurality of BTSs. The plurality of BTSs and the BSC is commonly referred to as a base station. And the wireless network may have a plurality of base stations. The plurality of base stations in the wireless network is referred to as a base station system (BSS).
Each BSC in the wireless network may communicate with a packet gateway and a mobile switching center (MSC). The packet gateway and the MSC function to set up and connect calls with other entities. For example, the packet gateway may set up and connect calls with a server or other entity on an Internet protocol (IP) network, and the MSC may set up and connect calls with a telephone on a public switched telephone network (PSTN).
Generally, the BSC will assign a traffic channel respectively to each wireless terminal for transmitting and receiving signals over the air interface. Additionally, the packet gateway establishes a radio-packet (R-P) link with the BSC. The R-P link carries signals between the packet gateway and the BSC. The packet gateway establishes a separate R-P link for each wireless terminal in the wireless network, and in this regard, the signals carried by each R-P link are associated with a particular wireless terminal. The R-P link is referred to as the A10/A11 link in the code division multiple access (CDMA) network architecture.
In some instances, a server in the wireless network may send data to wireless terminals in the wireless network. The data may represent information of interest to users of the wireless terminals, such as sports scores, weather reports, or advertising messages for instance. Alternatively, the data might comprise real-time streaming media such as voice or video conference content or broadcast television or radio signals. In any event, the server may transmit the data to one or more wireless terminals by inserting the data into one or more packets and transmitting the packets to the one or more wireless terminals.
An increasingly popular feature of wireless communications in recent years, and one which makes use of the above data transmission construct, is push-to-talk “walkie-talkie” service. With this feature, a user of a suitably equipped cell phone can “instantly” establish communication with one or more other users by simply pressing a PTT button on the cell phone. When the user presses the PTT button, the cell phone responsively transmits a PTT session initiation message (a type of data message) to a PTT server on the IP network, designating one or more target subscribers with whom to establish the session. In response, the PTT server then engages in further session initiation signaling with each target subscriber and with the initiating subscriber, to set up a PTT session leg respectively between the PTT server and each subscriber. The PTT server then bridges those session legs together so that the subscribers can communicate with each other, and so the session begins. A similar arrangement can also be used to establish instant video communication (“push-to-view” (PTV)), or other sorts of instant communication, generally known as “Push-to-x over Cellular” or “PoC”.
Unfortunately, however, one of the scarcest resources in a cellular communication system is the air interface between the BSS and the wireless devices. Typically, a given base station will be operate at a designated carrier frequency, or at one frequency on the “forward link” (from the base station to wireless devices) and at another frequency on the “reverse link” (from the wireless devices to the base station). Communications in a given cell sector may then be divided into channels by spread-spectrum modulation and/or time-division multiplexing, for instance. However, a limited number of such channels will exist.
Given this air interface limitation, most PoC systems are arranged to operate in a half-duplex mode, in which only one participant at a time has “the floor.” The participant with the floor is considered to be the “talker,” and each other participant is considered to be a “listener.” (These terms are not restricted to PTT or literally to talking and listening but more broadly apply to PoC.) In practice, the PoC server will transmit to each listener the bearer data (e.g., voice, video, etc.) that the PoC server receives from the talker. Further, the PoC server will apply a floor control process, through which the PoC server allows any party to take the floor as long as no other party currently holds the floor.
In a scenario where multiple listeners in a PoC session are located in a common cell sector, it is a relatively inefficient use of air interface resources for a PoC server to send bearer data from the talker over separate PoC session legs, and particularly over separate air interface traffic channels, one to each listener. It would be far more efficient in such a scenario if the PoC server would instead multicast the talker's bearer data to the listeners. That way, a single copy of the talker's bearer data could be sent to the base station serving the cell sector, and the single copy could be transmitted over a common multicast air interface channel for receipt concurrently by all of the listeners in the cell sector. At issue, however, is how to set up a PoC session in a manner that allows the participants to receive such multicast transmissions from the PoC server.